Process of electrodepositing insulative material on photoconductive copysheet



United States Patent 3,257,304 PROCESS OF ELECTRQDEPOSITING INSULA- TIVE MATERIAL 0N PHOTOCONDUCTIVE COPYSHEET Edgar G. Johnson, St. Paul, Minn., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware No Drawing. Filed Nov. 27, 1961, Ser. No. 155,178

11 Claims. (Cl. 204-181) This invention relates to the formation of permanent reproductions of light images on light-sensitive surfaces by electrolysis of -a highly photoconductive copysheet, after the exposure thereof. In one aspect, this invention relates to the electrolytic deposition of 'an electrolytically insulating material on the light exposed areas of a photoconductive copysheet. In still another aspect this invention relates to a photoconductive copysheet having selected areas of the photoconductive surface marked or electro-' lytically blocked by insulating deposits thereon.

The use of photosensitive sheet materials having surface layers which become electrically conductive when irradiated with light of certain wave lengths is known. Thus, a photoconductive material, such as zinc oxide, can be coated onto a sheet of electrically conductive material, exposed to a light image, and subjected to electrolysis in the presence of various reducible developer solutions, such as aqueous silver nitrate solution. The differential conductivity pattern produced by illumination with the light image is thereby electrolytically developed by reduction of the developer material to form a visible reproduction of the original light image. Such methods are disclosed in United States Patents Nos. 3,010,883 and 3,010,884. Although this process is capable of producing both positive as well as negative prints, various improvements have been desired in the production of positive prints and in the production of lithographic master copysheets.

It is therefore an object of this invention to provide an electrically insulating mask on the light exposed portions of a photoconductive copysheet.

A further object of this invention is to provide a means for electrolytically blocking previously light exposed areas of a photoconductive copysheet and for providing an electrolytically insulating coating or layer on said light exposed areas.

Another object of-this invention is to provide a novel lithographic master sheet and a method for its preparation and use.

Still another object of this invention is to provide a means for electro'ytically providing a positive print on a photoconductive copysheet which is exposed to a positive light image. i

Yet another object of this invention is to provide a novel process for reproducing light images on photoconductive copysheets without reversal, i.e. for providing positive prints from positive lightimages and negative prints from negative light images.

In accordance with this invention, a photosensitive sheet having a photoconductive particulate material, such as zinc oxide, indium oxide, cadmium sulfide, etc., is bonded to an electrically conductive backing, preferably by means of a suitable insulative binder, as described in United States Patents Nos. 3,010,883 and 3,010,884 and United States Serial No. 848,219, filed October 23, 1959. During or after exposure of the photoconductive layer to a light image to form a differential conductivity pattern thereon, the photoconductive sheet is developed by electrolytically depositing an insulative material selectively on the more conductive areas of the exposed copysheet in sufficient quantity to form an electrolytically insulative, Water insoluble, water impermeable coating. The insula- Patented June 21, 1966 tive coating material may be deposited from a solution or from a suspension or dispersion, depending on the solubility characteristics. Any insulative material which, after electrolytical deposition from solution, forms Water insoluble deposits on the copysheet can be used. Illustrative of such materials are the organic chelates or coordination compounds and amine derivatives, such as amine salts or quaternaries, including the Werner type chromium complexes of fatty acids, various fatty amine derivatives, and amino-containing resins. Any finely divided insulative material which can be suspended or dispersed in a current conducting medium, especially water, can also be employed in this invention to provide the insulative coating, although organic materials and particularly polymeric latices are preferred. Where charged particles rather than ions are current carriers, the process is generally referred to as electrophoresis although electrolysis as generically used herein is defined to include both charged particles and ions. The hydrogen ion and other ion concentration must not be at the isoelectric point for the particles of the suspension or dispersion, since at this value the particles have no net charge (i.e. the zeta potential is zero). Moreover, at the isoelectric point the colloidal suspension or dispersion generally has its minimum stability and electrical conductivity. Both positive and negatively charged particles may be employed, although the positively charged particles are preferred when used in conjunction with photoconductive zinc oxide copysheets because of the rectification tendencies of these sheets when the electrically conductive backings thereof are connected as anode. Illustrative of the electrophoretic developers containing positively charged insulative particles are the condensation products of polymerized unsaturated fatty acids (e.g. dilinoleic acid) with aliphatic amines (e.g. ethylene diamine), etc. Illustrative of the electrophoretic' developers containing neagtively charged insulative particles are polyethylene, polytetrafiuoroethylene, synthetic rubber (e.g., butadiene-acrylonitrile latex), polyvinyl acetate, polystyrene, butadiene-styrene copolymers, butadieneacrylic acid copolymers, natural rubber, polyvinylidene chloride, etc.

In certain instances it may also be desirable to incorporate certain filler materials into the dispersion or suspension, thereby to alter the properties of the deposited insulative film. Such fillers need not necessarily have an electrical charge, since the insulative charged particles can serve as carrier for these filler materials. Such fillers include dyes, carbon black, silica, etc.

When such deposited insulative materials are more hydrophobic than the photoconductive coating, the light struck areas are generally more oleophilic and ink receptive in relation to the more hyldrophilic non-light struck areas, thus forming a useful lithographic master sheet. However, the non-insulated or background areas may be subsequently treated to modify the hydrophilic properties and provide a greater contrast in such properties as compared to the insulative electrolytically deposited image areas, as will be further described later. 1

The receptor sheet which is exposed to the light image and on which the precipitate image' is formed contains an electrically conductive base, such as metal foil, upon Which a photoconductive layer of a material having a photoconductivity value of at least about 10- mho/ cm. and a dark conductivity value not greater than about onetwentieth of the photoconductivity value, is placed or bonded. The electrically conductive backing need not be does not adversely affect the light sensitivity of the photoconductive material. Such polymers include a 30:70 mol ratio copolymer of butadiene and styrene (30% solution in toluene), polystyrene, chlorinated rubber, rubber hydrochloride, etc. Polymers which are dissolved or softened by water, orwhich are dark in color, or insoluble in commercial solvents, or reactive with the photoconductive material, or which readily wet the photoconductive particles, are found to be less effective. Thus,

polyvinyl alcohol, polyacrylic acid, shellac and sodium carboxymethyl cellulose are generally not used as binders for the light-sensitive materials.

As light-sensitive materials, zinc oxide, cadmium sulfide, indium oxide, and other photoconductive powders having equivalent apparent photoconductivity values, as above described, have also been found to provide adequate photoconduc-tivity values in coated film form and to produce receptor sheets suitable for use in the instant invention. Mixtures of the photoconductive materials may also be employed. To improve or enhance the sensitivity of these photoconductive materials in certain visible and non-visible areas of the light spectrum, dye sensitizers, such as Acridine Orange, Fluorescein, Eosin Y, Rose Bengal, Methylene Blue, etc., are preferably admixed with the photoconductive powder.

Metal foil or sheet provides a suitable electrolytically conductive backing. Metal conductors, such as clean aluminum, chromium, nickel, silver and copper are suitable for the electrically conductive copysheet backing and may additionally be placed on the surface of a non-conductive supporting sheet, eg by vapor deposition, lamination, etc.

The photoconductive copysheets are exposed to a light image and, either simultaneous with or shortly thereafter, electrolytically developed by contacting the exposed surface with the solutions, suspensions or dispersions described earlier, the electrically conductive copysheet backing being connected so as to be opposite in electrical charge to the charge on the ions or charged particles to be deposited. The impressed voltage can be varied over a wide range and can readily be optimized for any given set of conditions. As the electrolytic development proceeds and the ions or charged particles reach on the light exposed surface areas, an insulative film or barrier is created thereon, and the current flow through these areas decreases accordingly. The resulting copysheet may be thereafter used as a lithographic master sheet as described earlier. If a print is desired without image reversal, such as a positive print from a positive transparency, the selectively insulated image areas of the copysheet being effectively blocked to further electrolytic development, the entire surface of the copysheet can be exposed to light and electrolyzed once again in the presence of a reducible metal ion, such as silver, nickel, etc. or a suitable dye. Some of such electrolytic developers for providing a dense dark background are discussed and illustrated in United States Patent No. 3,010,883. The dark background areas thus provided give good contrast with the lighter colored image areas, particularly when the image areas are blocked by the various insulative polymers deposited from aqueous latices. In still another embodiment, the background areas can also be darkened or colored by immersing the partially insulated copysheet into a coloring bath which selectively wets and colors the more hydrophilic background areas without affecting the color of the insulative coating on the image areas.

The following examples will illustrate the invention.

Example I A strongly photoconductive copysheet having a photo conductive zinc oxide-binder coating on an aluminum vapor coated polyethylene terephthalate film was exposed taining an aqueous cationic suspension of a polymer prepared by the condensation of polymerized unsaturated fatty acid and an aliphatic amine (average molecular weight 3000-6500, ball and ring softening point 43-53 C.) was slowly wiped over the copysheet surface. A transparent, water impermeable layer of polymer which was electrolytically insulative (relative to the uncoated areas) Was deposited in the light struck-areas. The entire copysheet surface was then exposed to light and developed electrolytically in similar manner with a sponge containing a dilute aqueous solution of silver nitrate and thiourea. Dark silver deposits were formed on those areas having no polymeric deposit from the prior development. A clear, high contrast positive copy of the original positive light image was obtained.

Example II A strongly photoconductive copysheet having a film of zinc oxide in a 30:70 mol ratio butadiene-styrene copolymeric binder (containing a minor amount of titanium dioxide) on an aluminum foil backing was taped to a Lucite block. This copysheet was then exposed for 30 seconds to 200 foot candles of incident light using a transparency to provide an image. The exposed copysheet was immediately placed in an electrolytic cell filled with a dilute anionic aqueous latex of a /10 weight ratio butadiene-acrylic acid copolymer, the aluminum layer of the copysheet being connected as cathode in this instance for a period of about 30 seconds, to condition the sheet. A high flow of from 0.4 to 0.5 ampere was noted (DC. voltage of 5-8 volts). The polarity was then reversed. with a resultant brief surge of anodic current of up 0.4 ampere. In less than one-half second the current fell to less than 0.04 and remained constant at that value for 30 seconds. A copolymeric insulative coating was thus anodically deposited on the light struck areas. The coated copysheet was withdrawn from the bath, flooded with light and electrolytically cathodically developed in a dilute aqueous solution of'0.66 wt. percent silver nitrate, 1.18 wt. percent of ethylene thiourea, 1.0 wt. percent acetic acid, and 5 wt. percent of magnesium acetate, thereby depositing dense dark silver in the polymer-free areas.

The anodic deposition of polymer can also be accomplished without cathodic conditioning before the anodic development either by intense exposure prior to the initial development or by simultaneous exposure and electrolytic development, thereby overcoming the rectification effect which normally tends to increase the resistance to current flow during anodic development.

Example III A strongly photoconductive copysheet having a photoconductive coating of Zinc oxide in a 30:70 mol ratio butadiene-styrene copolymeric binder on an aluminum foil backing was taped onto a Lucite block. With the aluminum backing connected as cathode the photo-sensitive copysheet surface was exposed to a light image, then immersed in an electrolytic bath consisting of a cationic aqueous latex of an approximate 90/10 weight ratio butadiene-acrylic acid copolymer. Using approximately 10-20 volts DC. a polymeric insulative coating was electrolytically deposited on the light struck areas. All steps subsequent to exposure to the light image were conducted in the dark. This copysheet can be further electrolytically developed to produce a positive image by deposition of a free metal in the polymer-free areas, as in Example II.

Various other embodiments of the present invention will be apparent to those skilled in the art without departing from the scope thereof.

I claim:

1. A process which comprises connecting as an electrode an electrolytically developable photoconductive PS et having a differential conductivity pattern thereon and electrodepositing by charged particle migration in an electrically conductive liquid medium a substantially water impermeable insulative material selectively onto the surface of said photoconductive copysheet to form an electrolytically insulating, water impermeable coating thereon.

2. The process of claim 1 in which said charged particles are ionic.

3. The process of claim 1 in which said charged particles are nonionic.

4. The-process of claim 1 in which said charged particles are positively charged and said photoconductor sheet is connected as cathode.

5. Thefpro'cess of claim 1 in which said charged particles are negatively charged and said photoconductor sheet is connected as anode.

6. A process which comprises connecting as an electrode an electrolytically developable photoconductive zinc oxide copysheet having a differential conductivity pattern thereon and electrodepositing by charged particle migration in an electrically conductive aqueous medium a substantially water impermeable, hydrophobic insulative material selectively onto the surface of said photoconductive zinc oxide copysheet to form an electrolytically insulating, Water impermeable coating thereon.

7. The process of claim 6 in which said insulative material has different wetting properties from said sur-.

face and subsequently using the resulting copysheet as a lithographic master.

8. The process of claim 6 in which said Water impermeable, hydrophobic insulative material is an organic polymer.

6 subsequently selectively coloring the uncoated portions of said surface.

10. The process of claim 9 in which said coloring is accomplished by contacting said surface with a dye that selectively wets the uncoated portions of said surface.

11. A process which comprises connecting as an electrode an electrolytically developable photoconductive copysheet having a differential conductivity pattern thereon, electrodepositing by charged particle migration in an electrically conductive liquid medium a substantially water impermeable, hydrophobic insulative material selectively on to the surface of said photoconductive copysheet to form an electrolytically insulating barrier coating thereon, uniformly exposing said copysheet to light, and electrolytically depositing free metal on the uncoated surface portions thereof with said copysheet being connected as cathode.

References Cited by the Examiner UNITED STATES PATENTS 1,804,920 5/ 1931 Edwards 204-182 2,276,986 3/ 1942 Kemp e-t al. 204-131 2,898,279 8/ 1959 Metcalfe et al.

2,993,787 7/1961 Sugarman.

3,010,883 11/1961 Johnson et a1. 204-18 3,053,688 9/1962 Greig 96-1 3,095,808 7/ 1963 Eastman 204-18 3,096,260 7/ 1963 Nail 204-18 3,104,169 9/1963 Metcalfe et al. 96-1 3,106,156 10/1963 Reithel 101-1492 OTHER REFERENCES Metcalfe et al.: Xerography in Journal of the Oil & Colour Assn. 39 (11), pp. 845-853, November 1956.

Shyne: Electrophoretic Application of Organic Finishes in Organic Finishing, vol. 17, No. -56, pp. 12-14.

WINSTON A. DOUGLAS, Primary Examiner. JOSEPH REBOLD, JOHN H. MACK, Examiners. R. L. GOOCH, A. B. CURTIS, Assistant Examiners. 

1. A PROCESS WHICH COMPRISES CONNECTING AS AN ELECTRODE AN ELECTROLYTICALLY DEVELOPABLE PHOTOCONDUCTIVE COPYSHEET HAVING A DIFFERENTIAL CONDUCTIVITY PATTERN THEREON AND ELECTRODEPOSITING BY CHARGED PARTICLE MIGRATION IN AN ELECTRICALLY CONDUCTIVE LIQUID MEDIUM A SUBSTANTIALLY WATER IMPERMEABLE INSULATIVE MATERIAL SELECTIVELY ONTO THE SURFACE OF SAID PHOTOCONDUCTIVE COPYSHEET TO FORM AN ELECTROLYTICALLY INSULATING, WATER IMPERMEABLE COATING THEREON. 